How to call one custom helper function in another custom helper

I want to use one helper function in another helper function. In below code I want to highlight last name if it contains "Finch" word. I have writtern helper class for that. If we use in hbs file then syntax would be {{highlight name}}. But how to use it since I have to use it in another helper class.

Below is my code:

  Handlebars.registerHelper('fullName', function(person) {
    return person.firstName + " " + person.lastName;
  });

   Handlebars.registerHelper('highlight', function(person) {    
var item = (person.lastName).replace('Finch', '<span style="color: red">' 
  + Finch + '</span>');
return new Handlebars.SafeString(item); 
  });

Here is the working fiddle : http://jsfiddle.net/wC6JT/4/

Here is the fiddle where "highlight" helper is called.: http://jsfiddle.net/wC6JT/3/ . This will not produce any results since we will get console errors for person.lastName not recognized in "highlight" register helper.

I want to use "highlight" helper in fullname helper for person.lastName. How can this be acheived.